The purposes of this study are to describe in biochemical terms how isolated mammalian cells interact with lipids contained in the extracellular fluid and to determine how the availability of extracellular lipids influences lipid synthesis within the cells. A comparison will be made between the lipoproteins present in Ehrlich ascites plasma and those present in mouse blood plasma. Cells will be incubated with lipoproteins containing labeled triglycerides, and the metabolism of the fatty acid and glycerol moieties will be determined. The ascites plasma will be examined for the presence of lipolytic enzymes in order to determine whether triglyceride utilization by Ehrlich cells possibly can occur through a mechanism in which the triglycerides are hydrolyzed prior to contact with the cells. Release of a lipase from the cells during in vitro incubation also will be examined. The transfer of cholesterol between cells and extra cellular fluid lipoproteins will be characterized, and we will determine the extent to which extracellular cholesterol is esterified after uptake by the cells.